This invention relates to a continuous process for the preparation of thymol by propylation of m-cresol with propylene.
It is already known to prepare thymol by reaction of m-cresol with propylene or compounds which eliminate propylene, such as oligomers of propylene, isopropanol, thymol isomers or poly-propylated m-cresol.
These alkylation reactions are carried out at temperatures of up to about 400.degree. C, under atmospheric pressure or, preferably, under a pressure of up to about 100 bars, and in the presence of catalysts. Examples of catalysts employed are catalysts of the Friedel-Craffts type, metal salts, aqueous solutions of metal salts, aluminium oxides, aluminium oxides pretreated with metal sulphates, natural and synthetic activated clays, alumosilicates, molecular sieves, treated natural products, such as bleaching earths, and cresolates (see, for example, East German Pat. No. 36,644, (German Published Specifications 1,815,846 and 2,139,622 and Angew. Chemie 1957, page 702).
However, these known processes suffer from the following serious disadvantages:
They can only be carried out discontinuously and/or require long reaction times; the catalysts used have too short a life. Or the catalysts require regenerating; the catalysts are difficult to separate off; the purity of the m-cresol to be employed has to conform to high standards (for example, the cresol must be dry).
Furthermore, the isolation of the pure thymol from the crude thymols obtained in the known processes presents very particular difficulties. The crude thymols obtained from the propylation of m-cresol in general contain 55 to 60 percent by weight of thymol, 25 to 30 percent by weight of unreacted m-cresol and 10 to 20 percent by weight of by-products. A decisive factor regarding the isolation of thymol from this crude thymol is the composition of the by-products. If these consist of compounds which can easily be separated from thymol, the isolation of the thymol presents no difficulties. If, on the other hand, the by-products contain compounds which hardly differ from thymol in their physical properties, especially their boiling point, for example 2-isopropyl-4-methyl-phenol, the preparation of the thymol in a pure form is, if not impossible, at least very expensive. 2,4-Diisopropyl-5-methyl-phenol is also an undesired by-product, since, when it is re-used as a propylating agent, it only reacts sluggishly.